In order to improve the aero-acoustic, vibratory, and aerodynamic performance of the blades of a main lift and advance rotor of a rotorcraft, it is advantageous to place a tiltable flap in the trailing edge of each blade, which flap is conveniently referred to as a trailing edge flap.
The angle through which the flap can be tilted relative to the blade is of the order of ±10° at a frequency of about 30 hertz (Hz). To be effective, the angle of tilt varies actively over time as a function of numerous parameters such as the position of the blade about the axis of the main rotor, referred to as the “azimuth” position, the speed of rotation of the main rotor, and the pitch, beating, and lag movements of the blade.
Given the dimensions of the blade and the stresses that act on the flap and the blade due to the beating, lag, and pitch movements thereof, it will readily be understood that there are difficulties to be overcome in obtaining a flap that is reliable, lightweight, and capable of satisfying the expectations of the manufacturer and of users.
Patent document US 2002/0021964 discloses a first type of blade provided with a flap. The top and bottom portions of the leading edge of the flap are convex in shape, while the central portion of the leading edge is concave in shape. The leading edge of the flap is thus ε-shaped.
In addition, the top and bottom portions are each connected to an actuator, while the concave central portion is placed against a bearing element secured to the blade. Consequently, by actuating one or the other of the actuators, the flap can be caused to tilt about the bearing point so as to present the desired inclination.
Nevertheless, under the effect of the stresses exerted on the blade and on the flap in flight, in particular those caused by beating movements, there is a risk of the flap sliding along the bearing point which could lead to premature wear, or even, in a worse-case scenario, to the flap becoming jammed, thereby making it unusable and dangerous.
Furthermore, it can readily be seen that the overall shape of the flap, and more particularly of its leading edge, is not optimized from an aerodynamic point of view and that might possibly lead to noise or vibration disturbance and to degraded aerodynamic performance.
U.S. Pat. No. 6,454,207 discloses a second type of blade having a trailing edge flap.
The flap is secured to the blade about a longitudinal shaft, i.e. extending along the span of the blade, which shaft passes through the blade. Consequently, the flap can pivot about the longitudinal shaft, being driven by a connecting rod secured to the pressure side of the flap via a crank, and itself controlled by an actuator. The connecting rod is thus situated in part outside the blade-and-flap assembly, which is harmful to overall aerodynamics. In addition, discontinuity is observed between the flap and the blade which can lead to a reduction in aerodynamic performance and also to a whistling effect due to air flowing between the flap and the blade.
In this second type of architecture, the means enabling the flap to pivot about a shaft still present the drawback of it being possible for the flap to jam. It should not be forgotten that the mechanical stresses acting on the flap and the blade in flight are very large. Consequently, there is a non-negligible risk of the shaft being deformed, and under such circumstances the flap could then no longer be tilted to the desired angle of inclination.
Finally, patent document FR 2 770 826 discloses a third type of blade fitted with a flap. The flap is not connected to the blade via a longitudinal shaft, but via two stub axles located on either side of the flap. In addition, the flap is provided with a flexible arm that is positioned inside the blade, being connected via a lever to a set of two rotary motors acting on coaxial eccentrics. Using that set of two rotary motors, the device controls the angle of inclination of the flexible arm, and thus of the flap.
Nevertheless, as for the second type of blade, the stub axles about which the flap tilts do not appear to present sufficient overall robustness to guarantee proper operation of the system over a reasonable length of time.